1. Field of the Invention
The present invention relates to a method for forming a coating film on an internal surface of an elongated tube and to a unit for forming the same. More particularly, the present invention relates to a method for forming a coating film on an internal surface of an elongated tube of a diameter of about 0.5 to 5 mm, the method allowing formation of a dried coating film, which is to serve as an electron emission layer, by performing, for example, heat treatment, and to a unit for forming the same.
2. Description of Related Art
As a display device, is known one in which a plurality of gas discharge tubes are arranged parallel to each other. This discharge device, using glass tubes of a diameter of about 0.5 to 5 mm, is so constructed that electrodes are formed outside the glass tubes; a discharge gas is enclosed in the glass tube to produce one gas discharge tube; and the plurality of gas discharge tubes are arranged in a row direction (or column direction) to constitute a display screen.
As such a display device, are known a large-scale gas discharge display panel described in Japanese Unexamined Patent Publication No. Sho 61(1986)-103187, an image-display device described in Japanese Unexamined Patent Publication No. Hei 11(1999)-162358 and the like. These display devices, as ones for large-scale display, are advantageous in reduced number of fabrication steps, reduced weight and costs, and ease of screen size change.
In the gas discharge tube used in the above-mentioned display devices, the electron emission layer is sometimes formed on a discharge surface, i.e., on the internal surface of the elongated tube, which is to serve as the gas discharge tube, for the purpose of improvement of the discharge characteristics such as lowering of a firing voltage. However, it is very difficult to form the electron emission layer on the internal surface of the elongated tube of a diameter of about 0.5 to 5 mm.
In the formation of the electron emission layer by deposition for example, molecules obtained by evaporation from a material introduced from an end of the elongated tube for forming the electron emission layer, deposit in a larger amount at an area nearer to the end of the elongated tube, and thus an uniform distribution of thickness is not achieved in the elongated tube. Nonuniformity of thickness of the electron emission layer causes variations of firing voltage at a plurality of emission points present in the elongated tube, resulting in a narrow margin of behavior for emission.
Accordingly, there has been demanded a method for forming a coating film, the method allowing easy formation of a dried coating film, which is to serve as the electron emission layer, by subjecting the internal surface of the elongated tube of a diameter of 0.5 to 5 mm to, for example, heat treatment.